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From the Department of Anesthesiology and Pain Medicine, Konkuk University school of Medicine, Seoul, Korea.
Address correspondence to: Dr. Duk-Kyung Kim, Department of Anesthesiology and Pain Medicine, Konkuk University School of Medicine, 1 Hwayang-Dong, Gwanggin-Gu, Seoul, 143-701, Korea. Phone: 82-2-2030-5442; Fax: 82-2-2030-5422; E-mail: dikei{at}kuh.ac.kr
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Abstract |
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Methods: In a prospective trial, 200 adult patients undergoing elective surgery under general anesthesia were randomly assigned to receive ventilation without airway warming and exogenous humidification (Group C - control), or active warming and humidification of inspired gases (Group HUM - humidified), using a humidifier with a heated wire circuit. Inhalational anesthesia was maintained via a circle system. The temperatures and relative humidities of inspired gases were monitored continuously throughout surgery. Postoperative sore throat, dysphonia, and dysphagia were assessed one and 24 hr after anesthesia. Whenever symptoms were present, their severities were graded using a 101-point numerical rating scale.
Results: The mean temperature and relative humidity of the inspired gases in Group HUM were greater compared to Group C (36.1 ± 0.4°C and 99.5 ± 0.5% vs 26.9 ± 0.8°C and 76.4 ± 10.9%, respectively). Postoperatively, the overall frequencies of laryngeal and pharyngeal discomfort were similar in the two groups (53.8% and 54.9% in Group C vs 51.6% and 41.9% in Group HUM at one and 24 hr respectively, P > 0.05). The groups were also similar with respect to the severity scores of laryngo-pharyngeal discomfort.
Conclusion: Active warming and humidification of inspired gases has no clinically appreciable effect in reducing the incidence and severity of laryngo-pharyngeal complaints after brief (< two hours) LMA anesthesia.
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Introduction |
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With endotracheal intubation and tracheostomy, normal upper airway mechanisms of warming and humidification of inspired gases are bypassed. The delivery of cold and dry medical gases through artificial airways can lead to a continuous loss of heat and moisture, thereby reducing mucociliary clearance, which may eventually lead to mucous membrane inflammation, ciliary damage, atelectasis, and impairment of pulmonary function.3,4 The extent of these airway responses will increase with increasing duration of ventilation. Therefore, provision of heat and humidity has become a standard of care for patients whose lungs are ventilated mechanically for a prolonged period.5,6 In contrast to the well-proven efficacy of humidification of the inspired gases in an intensive care unit setting, potential clinical benefits in patients ventilated mechanically during general anesthesia, especially for a relatively short duration (< two hours), remain controversial.
Recently, the LMA has become a preferred airway device for minor surgeries of relatively short duration, lasting one to two hours. Compared to an endotracheal tube, the LMA permits exposure of a larger segment of the airway to the effects of inhaled anesthetics because of its anatomic position within the hypopharynx. Despite its widespread use, there have been no prospective studies which have specifically examined the clinical efficacy of humidification of inspired gases in reducing laryngo-pharygeal complaints after LMA-based general anesthesia. We therefore hypothesized that the efficiency of humidifying inspired gases during LMA anesthesia may be attenuated compared to that observed during general anesthesia with an endotracheal tube. In this prospective study, we sought to determine whether active warming and humidification of inspired gases would reduce the incidence and severity of postoperative laryngo-pharyngeal complaints in otherwise healthy patients after brief LMA anesthesia.
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Methods |
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Patients were unpremedicated. In the operating room, prior to induction, patients in Group HUM had a servo-controlled heated humidifier adjusted to maintain a distal inspiratory circuit temperature of 37°C. Following application of routine monitors, anesthesia was induced with thiopental 5 mg·kg–1 iv and vecuronium 0.1 mg·kg–1 iv. Four anesthesiologists, each with experience in over 100 previous LMA insertions, attended to airway management. Before insertion, the LMA cuff was fully deflated and clear, water-based lubricant (K-Y Lubricating Jelly, Johnson and Johnson, Maidenhead, UK) was applied to the dorsal surface of the airway device. Size 4 LMAs (LMA-ClassicTM, Intavent Orthofix, Maidenhead, UK) and size 5 LMAs were used for female and male patients, respectively.
After insertion, the LMA cuff was inflated with air to the maximum recommended volume (size 4: 30 mL; size 5: 40 mL). The cuff was then gradually deflated in 2-mL decrements until a ‘sealing pressure’ at 20 cm H2O was obtained. The absence of an audible leak at 20 cm H2O inflation pressure, synchronized expansion of the chest on positive pressure ventilation, and a normal capnographic trace ascertained an adequate sealing pressure. This was the minimum LMA cuff pressure that provided efficient positive pressure ventilation in most surgical patients, based on our experience. The initial cuff pressure was documented using an airway pressure manometer (control inflator, VBM Medizintechnik GmbH, Sulz and Neckar, Germany). If the insertion attempt failed, or if the airway was ineffective, the LMA was reinserted. If LMA insertion was unsuccessful after two attempts, the patient’s trachea was intubated, and the subject was withdrawn from the study. The difficulty of LMA insertion was graded by the anesthesiologists according to the following criteria: 1 (one attempt at insertion with no tactile resistance); 2 (one attempt at insertion with some advancing difficulty); or 3 (two attempts at insertion).
Anesthesia was maintained with sevoflurane in combination with 67% air in oxygen via a circle anesthesia breathing system (Aestiva/5, Datex-Ohmeda, Helsinki, Finland). Patients were mechanically ventilated with 8 mL·kg–1 tidal volume at a respiratory rate of 10–14·min–1 to maintain a PETCO2 of 30–40 mmHg, at an I:E ratio of 1:2, with a fresh gas flow rate of 3 L·min–1. After a five-minute period for stabilization, we measured temperature and relative humidity of the inspired gases using a thermohygrometer (SK-110TRH, Sato Keiryoki Mfg Co., Tokyo, Japan), placed between the Y-piece and distal end of the LMA. Its accuracy was ± 0.5 °C temperature and ± 2% relative humidity at 20 to 40°C. These values were continuously monitored and recorded at ten-minute intervals throughout the case. Additionally, absolute humidity was calculated on the basis of temperature and relative humidity measurements by the formula:7
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Where AHS (mg H2O·L–1) = absolute humidity at saturation point, AH (mg H2O·L–1) = absolute humidity of inspired gas at measured gas temperature, RH (%) = relative humidity, and T (°C) = temperature of the inspired gas.
Ambient temperature and relative humidity also were measured using the thermohygrometer placed near the patients. These values were averaged over the entire intraoperative course, using measurements taken at ten-minute intervals. Approximately 15 min before the end of operation, patients received morphine 3–5 mg iv. After the surgical procedure was finished, anesthesia was discontinued and residual neuromuscular block was reversed. The LMA was removed with the cuff inflated when the patient was able to open her/his mouth to command. After its removal, the LMA was inspected for the presence or absence of blood on any of its surfaces.
Postoperative pain management in the postanesthetic care unit (PACU) and the ward were standardized. Morphine 2–3 mg iv prn boluses were administered for pain in the PACU, and ketolorac 15 mg iv prn was administered on the ward. Both the study subjects and the research assistants were blinded to group allocation. A trained research assistant interviewed the patients at one and 24 hr postoperatively. During the interview, the patients were specifically questioned about the presence or absence of the following symptoms: sore throat (constant pain, independent of swallowing), dysphonia (difficulty or pain on speaking), and dysphagia (difficulty with or pain provoked by swallowing). The severity of these symptoms was assessed using a 101-point numerical rating scale (0 = no discomfort, 100 = maximum discomfort).
The primary outcome was the postoperative 24-hr incidence of laryngo-pharyngeal complaints. We projected this incidence to be 40% in Group C, based on the findings of several previous studies.1,2 We considered a 50% reduction of complaints in Group HUM would be clinically significant. Assuming a statistical power of 80% at an alpha level of 0.05, for a clinical study design incorporating two equally-sized groups, we estimated that 91 patients would be required per group. To account for potential drop-outs or incomplete follow-up for some subjects, we planned to enrol 200 patients. Categorical variables were analyzed using Pearson 
2-tests with continuity correction or Fisher’s exact tests where applicable. Continuous variables were analyzed using unpaired two-tailed t tests. Statistical analysis was performed with SPSS 12.0 for windows (SPSS Inc., Chicago, IL, USA). Data are presented as mean ± SD. Significance was defined as P < 0.05.
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Results |
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Patient characteristics were similar in the two groups, and there were no differences between groups with respect to temperatures and relative humidities of the operating room, duration of anesthesia, and postoperative analgesic consumption (Table I
). The groups were comparable with respect to the success rate on the first attempt of LMA insertion, mucosal trauma assessed by traces of blood on the LMA, and the difficulty of LMA insertion, as rated by the anesthesiologists. Cuff pressures and volumes were similar between groups (Table II). No intraoperative ventilatory problems were observed in any patient, and there were no clinically significant complications associated with removal of the LMA in either group.
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). The severity of all symptoms at one and 24 hr postoperatively, assessed on the 101-point numerical rating scale, did not differ between groups (Table IV).
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Discussion |
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Many factors contribute to postoperative laryngo-pharyngeal morbidity associated with use of the LMA. In addition to the interview method used for data collection, 8 direct trauma to soft tissues,9,10 size of LMA,11 inflated volume and pressure of LMA cuff,1 duration of anesthesia,2 gender,2,10 and the mode of ventilation (spontaneous or controlled)12 may all influence the reported frequencies of postoperative symptoms. Dry and cold inspired gases are additional factors that have been suspected to influence the incidence and severity of postoperative laryngo-pharyngeal complaints.
Much of the discomfort after LMA-based anesthesia may be due to trauma and pain attributable to the insertion technique. To control for this factor, we made a concerted effort to minimize direct trauma to the soft tissues during LMA insertion by ensuring that a limited number of anesthesiologists skilled with the LMA were involved in airway management. Furthermore, we standardized the anesthetic technique. As a result, the groups were similar with respect to the number of insertion attempts, grade of insertion difficulty, and the incidence of blood on the LMA after removal. Also, the pressure of the LMA cuff on the pharyngeal mucosa may be responsible for the discomfort. Theoretically, the calculated pressure exerted by the LMA cuff on the pharyngeal wall may exceed the perfusion pressure of the mucosa in some cases. These conditions, in turn, may possibly contribute to postoperative complaints of sore throat. In the present study, mean intra-cuff LMA pressures were similar in the two groups.
The negative finding of the present study is most probably related to the relatively short duration of anesthesia. Ciliated pseudostratified respiratory epithelium and the stratified squamous epithelium which cover the laryngo-pharyngeal mucosa are relatively resistant to minor injury.13 Williams et al.14 suggested that at constant suboptimal humidity, an increasing exposure time increases mucosal dysfunction. Asmundsson et al.15 also suggested that irreversible cessation of ciliary activity leading to inflammation and sloughing of the upper airway mucosa requires an exposure time to dry gases of more than three hours.
The calculated mean absolute humidity of inspired gases in Group C was 19.6 ± 3.6 mg H2O·L–1. This value was considerably higher than the < 10 mg H2O·L–1 value of dry and room temperature anesthetic gases. This observation might be explained in part, by the effects of the circle absorber system. In Group C patients, the amount of humidity in the inspired gas is relative to the proportion of rebreathed gas to fresh gas. Therefore, Group C patients may not have experienced a humidity deficit of sufficient magnitude to influence the upper airway mucosa over a one- to two-hour exposure period. In an animal model, Dias et al.16 found that both temperature and absolute humidity of tracheal air in their LMA group were significantly higher than observed in an endotracheal tube group (23 mg H2O·L–1 vs 14 mg H2O·L–1, P < 0.0001). These investigators suggested that use of the LMA might expose a larger segment of the upper airway (larynx and cervical trachea) to the conditioning effects of inspired gases compared to an endotracheal tube. Therefore, ventilation with an LMA could potentially deliver higher water vapour content than an endotracheal tube. In the present study, even if airway humidification and warming had not been applied in Group C, it was assumed that the inspired humidity was increased to some degree by the combined effects of the circle absorber system and use of the LMA.
As laryngo-pharyngeal discomfort lasting more than 24 hr could potentially delay discharge from hospital, we considered the postoperative 24-hr incidence of laryngo-pharyngeal complaints as the primary outcome. The frequencies of such complaints (54.9% in Group C and 41.9% in Group HUM) were higher than reported by other investigators.1,2,11,12 This may be related to the method employed for standardization of volumes and pressures of the LMA cuff. In the present study, the identical ‘sealing pressure at 20 cm H2O’ was applied to all patients instead of ‘just-seal pressure’, which has been strongly recommended by the manufacturers. However, other studies were done according to a ‘just-seal pressure’2,11 or fixed cuff volume.1,12 Our interview method may be also related to the higher rates of laryngo-pharyngeal complaints. It has been shown that leading questions, as adopted in this study, result in significantly higher detection rates than non-leading questions.8
Interestingly, active airway warming and humidification did not reduce both postoperative one- and 24- hr incidences of dysphonia. We initially doubted that the inhaled cold and dry gases might substantially contribute to the development of dysphonia because the LMA does not transverse the vocal cords. Considering our results together with those of Figueredo et al.12 who found that the incidence of dysphonia was greater in groups receiving mechanical vs spontaneous ventilation (17.2% vs 5%, P < 0.05) suggests that positive pressure ventilation per se, may have a greater influence on transient dysphonia than conditioning of the airway with humidification.
Our study must be interpreted with some caution. First, our results apply only to healthy adults undergoing brief LMA anesthesia. Patients with a compromised upper airway may have altered responses to cold and dry inspired gases. Williams et al.14 suggested that critically ill patients have other systemic demands that make individuals less tolerant to water mass and thermal challenges to their airway mucosa. Secondly, there are occasionally limitations in applying positive pressure ventilation without use of neuromuscular blockers, which has become more popular with the use of LMA. Recently, Fabregat and De Arce17 suggested that repetitive exposure to cold fresh gas flow may cause pressure-induced micro-trauma to partially- abducted or non-paralyzed vocal cords in such situations. Third, although our study showed that the incidence of dysphagia was proportionately less in Group HUM (48.4% vs 30.1% at 24 hr), retrospective power analysis of this data revealed that at least 247 patients per group would be required to achieve a power > 0.8 (P = 0.05) for detecting a statistically significant difference.
The routine use of heated humidifiers during general anesthesia might pose several problems. While ventilator associated pneumonia18 is unlikely following a brief exposure to exogenous humidification, a more common problem is that water condensates may compromise the function of ventilators and respiratory gas monitors. Another important consideration is the financial consideration of capital expenditures and maintenance costs if heated wire circuit were used routinely for anesthesia.
In conclusion, we have shown that active warming and humidification of inspired gases, at near core temperature (37°C) and saturated with water vapour (100% relative humidity), fails to reduce the incidence and severity of laryngo-pharyngeal complaints in healthy patients ventilated mechanically with use of an LMA for a relatively short duration (< two hours). Considering the lack of documented efficacy of heated humidifiers to reduce postoperative laryngo-pharyngeal morbidity, routine use of these devices in this setting does not appear to be warranted.
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Acknowledgments |
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Footnotes |
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Accepted for publication November 1, 2006. Revision accepted November 10, 2006.
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References |
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2 Rieger A, Brunne B, Hass I, et al. Laryngo-pharyngeal complaints following laryngeal mask airway and endotracheal intubation. J Clin Anesth 1997; 9: 42–7.[Medline]
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18 Kirton OC, DeHaven B, Morgan J, Morejon O, Civetta J. A prospective, randomized comparison of an inline heat moisture exchange filter and heated wire humidifiers: rates of ventilator-associated early-onset (community-acquired) or late-onset (hospital-acquired) pneumonia and incidence of endotracheal tube occlusion. Chest 1997; 112: 1055–9.
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